<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<body>

</body>
</html>
<script>
    setInterval(fn,1000);
    function fn() {
        花费时间2秒
    }
    setTimeout(fn,1000)
    function fn() {
    }
</script>